Filmy graphites are important as industrial materials because of their excellent heat resistance, chemical resistance, high thermal conductivity, and high electrical conductivity, and are widely used as heat-dissipating materials, heat-resistant sealing materials, gaskets, heating elements, etc.
As a representative example of a process for producing an artificial filmy graphite, a process referred to as an “expanded graphite production process” is known. In this process, natural graphite is dipped in a mixed solution of concentrated sulfuric acid and concentrated nitric acid, followed by rapid heating to produce an artificial graphite. The resulting artificial graphite is washed to remove the acids and then formed into a film with a high-pressure press. However, in the filmy graphite thus produced, strength is low and other physical properties are insufficient. Moreover, the residual acids also give rise to a problem.
In order to overcome these problems, a process has been developed in which a special polymer film is graphitized by direct heat treatment (hereinafter, referred to as a “polymer graphitization process”). Examples of the polymer film used for this purpose include films containing polyoxadiazole, polyimide, polyphenylenevinylene, polybenzimidazole, polybenzoxazole, polythiazole, or polyamide. The polymer graphitization process is a process which is far simpler than the conventional expanded graphite production process, in which mixture of impurities, such as acids, does not essentially occur, and which is capable of achieving excellent thermal conductivity and electrical conductivity close to those of single crystal graphite (refer to Japanese Unexamined Patent Application Publication Nos. 60-181129, 7-109171,and 61-275116).
However, the polymer graphitization process has two problems. First, it is difficult to obtain a thick filmy graphite compared with the expanded graphite production process. Although various attempts have been made to improve such a problem, as it now stands, transformation into a quality graphite is possible only when the thickness of the starting material film is up to about 50 μm.
Secondly, the graphitization requires long-time heat treatment at extremely high temperatures. In general, transformation into a quality graphite requires heat treatment in a temperature range of 2,800° C. or higher for at least 30 minutes.